Active solid-state devices (e.g. – transistors – solid-state diode – Test or calibration structure
Reexamination Certificate
2001-11-08
2004-12-14
Thompson, Craig A. (Department: 2813)
Active solid-state devices (e.g., transistors, solid-state diode
Test or calibration structure
C257S088000, C257S091000, C349S038000, C349S042000, C349S044000
Reexamination Certificate
active
06831295
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a TFT-LCD (thin-film-transistor liquid-crystal-display) device having a reduce feed-through voltage and, more particularly, to a TFT-LCD device which is capable of reducing variance or scattering in the parasitic capacitances between the pixel electrodes and the signal lines in the TFT-LCD device to reduce the feed-through voltage thereof.
(b) Description of the Related Art
In recent years, LCD devices attract higher attention as the flat display panels which are capable of reducing dimensions thereof and lowering power consumption. In particular, among the LCD devices, TFT-LCD devices are widely used in a variety of office equipment or video display devices due to the advantages thereof wherein gray-scale-level display can be obtained by different driving voltages, and a fine image is obtained thereon with reduced cross-talk between adjacent pixels.
FIG. 1
shows one of the pixels in a conventional active-matrix TFT-LCD device. The TFT-LCD device includes a TFT panel
10
made of glass and mounting thereon a plurality of pixels
11
arranged in an array and each including a TFT (thin film transistor)
12
and an associated pixel electrode
13
.
The TFT panel
10
further mounts thereon a plurality of scanning lines
14
each extending in a row direction of the array, a plurality of data lines
15
each extending in a column direction, and a plurality of light-shield members
16
disposed between adjacent pixels for shielding the light passing between the pixels
11
. A counter panel (not shown) mounting thereon a counter electrode opposes the TFT panel
10
, with a liquid crystal layer sandwiched therebetween. The scanning line
14
has a stripe extension
18
which constitutes a gate electrode of each TFT
12
, and a large width expansion
19
opposing the pixel electrode
13
of the adjacent pixel
11
disposed in the next row. The data line
15
has a stripe extension
22
which constitutes a drain of each TFT
12
, the source
21
of which is connected to the pixel electrode
13
.
The pixel electrode
13
and a corresponding portion of the counter electrode forms a LC capacitance (or LC capacitor), with the liquid crystal layer being a capacitor insulator. The pixel electrode
13
also forms a storage capacitor in association with the large width expansion
19
of the adjacent scanning line
14
.
FIG. 2
shows a pixel
11
of another conventional TFT-LCD device which is similar to the pixel
11
of
FIG. 2
except for a common line
27
extending adjacent to and parallel to one of the scanning lines
14
and having an expansion
26
instead of the expansion
19
of the scanning line
14
shown in FIG.
1
.
In operation of the TFT-LCDs shown in
FIGS. 1 and 2
, a gray-scale-level voltage is applied between the pixel electrode
13
and the counter electrode to store electric charge on the LC capacitor and the storage capacitor, thereby controlling the electrochemical characteristics of the liquid crystal between the pixel electrode
13
and the counter electrode. This controls the transmission of light through the liquid crystal layer and forms an image pixel by pixel on the LCD panel.
The TFT-LCD has a plurality of parasitic capacitances among the electrodes
13
, signal lines
15
, and light-shield members
16
, in addition to the pixel capacitor and the storage capacitor as described above, due to the complicated arrangement of the electrodes and the signal lines. The parasitic capacitances may vary significantly between the pixels and thus generate variance in the image on the display panel to affect the display performance of the TFT-LCD device.
FIGS. 3A and 3B
show schematic views of the TFT-LCD of
FIG. 1
, for example, for showing the variance in the parasitic capacitances. As shown in the figures, the parasitic capacitances are formed between the pixel electrode
13
(third layer) and the signal lines
15
(second layer) and between the pixel electrode
13
(third layer) and the signal lines
15
as well as the light-shield members
16
(first layer).
The TFT-LCD of
FIG. 3A
has an ideal alignment between these three layers formed on the TFT panel
10
, the ideal alignment providing a symmetry of the parasitic capacitances, such as Cdpi, between the right side and left side of the pixel electrode
13
. On the other hand, TFT-LCD of
FIG. 3B
has a misalignment between three layers on the TFT panel due to the photolithographic process, and thus has an asymmetry of the parasitic capacitances between the right side and the left side of the pixel electrode
13
, thereby increasing the feed-through voltage and degrading the display performance of the TFT-LCD device, as detailed below.
It is usual that the polarity of the pixel electrode is reversed with respect to the counter electrode at each frame for suppressing the burning of the LCD panel to improve the display performance. The reversing driving schemes include: a drain line reversing scheme wherein the pixels arranged in the adjacent columns have opposite polarities, with the pixels arranged in the same column having the same polarity, and the pixels in each column are reversed in the polarity thereof at each frame; and a dot reversing scheme wherein every two adjacent pixels have opposite polarities and are reversed in the polarity thereof at each frame.
The potential fluctuation of the data lines is highest at the time of reversion of the polarity thereof to vary the potential of the corresponding pixel electrodes, thereby causing a fluctuation of the brightness of the display. The reversing driving scheme cancels the brightness fluctuation between the adjacent data lines during the polarity reversion.
The amount of the fluctuation canceling may be limited, however, if a significant asymmetry of the parasitic capacitance resides between the adjacent data lines due to the misalignment of the conductive layers as described before. The asymmetry of the parasitic capacitance between the data lines is also caused by the arrangement of the TFT
12
in the pixel, which necessitates a provision of a cutout
36
in the pixel electrode
13
in the vicinity of the data line
15
.
Patent Publication JP-A-2000-98427 describes a TFT-LCD device which is capable of alleviating the brightness fluctuation of the LCD panel due to the voltage fluctuation of the data lines. The TFT-LCD device described therein has a symmetry of parasitic capacitance between the right side and the left side of the pixel electrode by equalizing the lengths of the portions of the two adjacent data lines extending parallel to and adjacent to the periphery of the pixel electrode.
In the TFT-LCD device described in JP-A-2000-98427, variance in the parasitic capacitance between the pixel electrode and the first conductive layer is not considered. In addition, the structure of the pixel electrode for equalizing the lengths reduces the effective pixel area for the image.
Patent Publication JP-A-6-222392 describes an active-matrix LCD device which is capable of suppressing variance in the parasitic capacitance between active elements without necessitating a high-accuracy mask alignment.
FIG. 4A
shows a top plan view of the active matrix LCD device described therein, and
FIG. 4B
shows a sectional view taken along line B—B in FIG.
4
A.
The LCD device has a plurality of pixels
30
arranged in a matrix and each including a square pixel electrode
31
, a plurality of scanning electrodes
32
each defining a shape of ladder having a frame section which surrounds a corresponding one of the pixel electrodes
31
, and a ring electrode
33
interposed between the pixel electrode
31
and the frame section of the scanning electrode
32
. As shown in
FIG. 4B
, the ring electrode
33
has an inner edge underlying the pixel electrode
31
and an outer edge overlying the frame section of the scanning electrode
32
with an intervention of a dielectric film
34
. The pixel electrode
31
is made of a transparent metal oxide (ITO) film which is difficult to pattern with an accura
Hogans David L.
Katten Muchin ZavisRosenman
NEC LCD Technologies Ltd.
Thompson Craig A.
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